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Volume (computing)
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In computer data storage, a volume or logical drive is a single accessible storage area with a single file system, typically (though not necessarily) resident on a single partition of a hard disk (so-called simple volume). Although a volume might be different from a physical disk drive, it can still be accessed with an operating system's logical interface. However, a volume differs from a partition.
Differences from partition
[edit]A volume is not the same thing as a partition. For example, a floppy disk might be accessible as a volume, even though it does not contain a partition, as floppy disks cannot be partitioned with most modern computer software. Also, an OS can recognize a partition without recognizing any volume associated with it, as when the OS cannot interpret the filesystem stored there. This situation occurs, for example, when Windows NT-based OSes encounter disks with non-Microsoft OS partitions, such as the ext4 filesystem commonly used with Linux. Another example occurs in the Intel world with the "Extended Partition". While these are partitions, they cannot contain a filesystem directly. Instead, "logical drives" (also known as volumes) must be created within them. This is also the case with NetWare volumes residing inside of a single partition. In short, volumes exist at the logical OS level, and partitions exist at the physical, media specific level. Sometimes there is a one-to-one correspondence, but this is not guaranteed.
In Microsoft Windows Server 2008 and onward, the term "volume" is used as a superset that includes "partition" as well.[1][2][3]
It isn't uncommon to see a volume packed into a single file. Examples include ISO9660 disc images (CD/DVD images, commonly called "ISOs"), and installer volumes for Mac OS X (DMGs). As these volumes are files which reside within another volume, they certainly are not partitions.
Example
[edit]This example concerns a Windows XP system with two physical hard disks. The first hard disk has two partitions, the second has only one. The first partition of the first hard disk contains the operating system. Mount points have been left at defaults.
| Physical disk | Partition | Filesystem | Drive letter |
|---|---|---|---|
| Hard Disk 1 | Partition 1 | NTFS | C: |
| Partition 2 | FAT32 | D: | |
| Hard Disk 2 | Partition 1 | FAT32 | E: |
In this example,
- "C:", "D:", and "E:" are volumes.
- Hard Disk 1 and Hard Disk 2 are physical disks.
- Any of these can be called a "drive".
Nomenclature
[edit]In Linux systems, volumes are usually handled by the Logical Volume Manager or the Enterprise Volume Management System and manipulated using mount(8). In NT-based versions of Microsoft Windows, volumes are handled by the kernel and managed using the Disk Management MMC snap-in or the Diskpart command line tool.
Windows NT-based operating systems
[edit]Windows NT-based OSes do not have a single root directory. As a result, Windows will assign at least one path to each mounted volume, which will take one of two forms:
- A drive letter, in the form of a single letter followed by a colon, such as "F:"
- A mount-point on an NTFS volume having a drive letter, such as "
C:\Music"
In these two examples, a file called "Track 1.mp3" stored in the root directory of the mounted volume could be referred to as "F:\Track 1.mp3" or "C:\Music\Track 1.mp3", respectively.
In order to assign a mount point for a volume as a path within another volume, the following criteria must be met:
- The mounted-to volume must be formatted NTFS.
- A directory must exist at the root path. (As of Windows Vista, it can be any subdirectory in a volume)
- That directory must be empty.
By default, Windows will assign drive letters to all drives, as follows:
- "A:" and "B:" to floppy disk drives, whether present or not
- "C:" and subsequent letters, as needed, to:
- Hard disks
- Removable disks, including optical media (e.g. CDs and DVDs)
Because of this legacy convention, the operating system startup drive is still most commonly assigned "C:", however this is not always the case. Since personal computers now no longer include floppies, and optical disc and other removable drives typically still start at "D:", letters A and B are available for manual assignment by a user with administrative privileges. This assignment will be remembered by the same OS on the same PC next time a removable volume is inserted, as long as there are no conflicts, and as long as the removable drive has not been reformatted on another computer (which changes its volume serial number), and as long as the OS has not been reinstalled on the computer.
On Windows XP, mount points may be managed through the Disk Management snap-in for the Microsoft Management Console. This can be most conveniently accessed through "Computer Management" in the "Administrative Tools" section of the Control Panel.
More than one drive letter can refer to a single volume, as when using the SUBST command.
Warning: removing drive letters or mount-points for a drive may break some programs, as some files may not be accessible under the known path. For example, if a program is installed at "D:\Program Files\Some Program", it may expect to find its data files at "D:\Program Files\Some Program\Data". If the logical disk previously called "D:" has its drive letter changed to "E:", "Some Program" won't be able to find its data at "D:\Program Files\Some Program\Data", since the drive letter "D:" no longer represents that volume.
Unix-like operating systems
[edit]In Unix-like operating systems, volumes other than the boot volume have a mount-point somewhere within the filesystem, represented by a path. Logically, the directory tree stored on the volume is grafted in at the mountpoint. By convention, mount-points will often be placed in a directory called '/mnt', though '/media' and other terms are sometimes used.
To use a given path as a mount-point for another volume, a directory (sometimes called a "folder") must exist there.
Unix-like operating systems use the mount command to manipulate mount points for volumes.
For example, if a CD-ROM drive containing a text file called 'info.txt' was mounted at '/mnt/iso9660', the text file would be accessible at '/mnt/iso9660/info.txt'.
Data management speed
[edit]Files within a volume can generally be moved to any other place within that volume by manipulating the filesystem, without moving the actual data. However, if a file is to be moved outside the volume, the data itself must be relocated, which is a much more expensive operation.
In order to better visualize this concept, one might consider the example of a large library. If a non-fiction work is originally classified as having the subject "plants", but then has to be moved to the subject "flora", one does not need to refile the book, whose position on the shelf would be static, but rather, one needs only to replace the index card. However, to move the book to another library, adjusting index cards alone is insufficient. The entire book must be moved.
Labels and serial numbers
[edit]
A volume label is the name given to a specific volume in a filesystem. In the FAT filesystem, the volume label was traditionally restricted to 11 characters (reflecting the 8.3 restrictions, but not divided into name and extension fields) even when long file name was enabled, stored as an entry within a disk's root directory with a special volume-label attribute bit set, and also copied to an 11-byte field within the Extended BIOS Parameter Block of the disk's boot sector. The label is always stored as uppercase in FAT and VFAT filesystems, and cannot contain special characters that are also disallowed for regular filenames. In the NTFS filesystem, the length of its volume label is restricted to 32 characters, and can include lowercase characters and even Unicode. In the exFAT filsystem, the length of its volume label is also restricted to 11 characters, but can include lowercase characters and Unicode. The label command is used to change the label in DOS, Windows, and OS/2. For GUI systems like Windows Explorer, F2 can be pressed while the volume is highlighted, or a right-click on the name will bring up a context menu that allows it to be renamed, either of which is the same process as for renaming a file. Changing the label in Windows will also change the volume creation timestamp to the current date and time for FAT filesystems. NTFS partitions have the System Volume Information directory, whose creation timestamp is set when Windows creates the partition, or when it first recognizes a repartitioning (the creation of a new volume) by a separate disk utility.
In contrast to the label, the volume serial number is generally unique and is not normally changed by the user, and thus acts as a more consistent and reliable identifier of when a volume has been changed (as when a disk is removed and another inserted). Disk formatting changes the serial number, but relabeling does not.[4] It originated in 1950s in mainframe computer operating systems. In OS/360 line it is human-configurable, has a maximum length of six characters, is in uppercase, must start with a letter, and identifies a volume to the system in unique manner. For example, "SYSRES" is often used for a system residence volume. Operating systems may use the volume serial number as mountpoint name.[5]
A volume serial number is a serial number assigned to a disk volume or tape volume. In FAT and NTFS file systems, a volume serial number is a feature used to determine if a disk is present in a drive or not, and to detect if it was exchanged with another one. This identification system was created by Microsoft and IBM during their development of OS/2.[6] It was introduced in MS-DOS 4.01 in 1988.
The volume serial number is a 32-bit number determined by the date and time on the real-time clock on the current computer at the time of a disk's formatting. Previously, determination by the OS of whether a disk was swapped was done by reading the drive's volume label. However, even at that time the volume label was not required to be unique and was optional. Therefore, many users had not given disks any meaningful name and the old method failed.
The vol command can be used from the command line to display the current label and serial number of a volume.
References
[edit]- ^
"Understanding Disk Terminology". Microsoft Corporation. Retrieved 2014-06-10.
Partition A portion of the hard disk. In many cases, this is the entire hard disk space, but it needn't be. Volume A unit of disk space composed of one or more sections of one or more disks. Prior versions of Windows Server used volume only when referring to dynamic disks, but Windows Server 2008 uses it to mean partitions as well.
- ^
"Partitions and Volumes". Microsoft Corporation. Retrieved 2014-06-10.
In Windows Server 2008 the distinction between volumes and partitions is somewhat murky. When using Disk Management, a regular partition on a basic disk is called a simple volume, even though technically a simple volume requires that the disk be a dynamic disk.
- ^
"Use Built-In Tools to Create Partitions and Volumes in Windows Server". Microsoft Corporation. 2008-06-18. Retrieved 2014-06-10.
Windows Server 2008 simplifies the Disk Management user interface by using one set of dialog boxes and wizards for both partitions and volumes.
- ^ Glass, Brett (1998-02-06). "Changing a Disk's Volume Serial Number". Brett Glass To The Rescue. Retrieved 2006-07-28.
- ^ "Issue with .XPSF playlists".
- ^ Letwin, Gordon (1988). Inside OS/2. Microsoft Press. ISBN 1-55615-117-9.
External links
[edit]- MSDN's article on Hard Links and Junctions
Volume (computing)
View on GrokipediaDefinition and Fundamentals
Core Concept
In computing, a volume refers to an identifiable unit of data storage managed by an operating system, which presents itself as a cohesive entity to users and applications regardless of any underlying physical or virtual subdivisions. This unit can encompass an entire storage device, such as a hard disk drive, or a portion thereof, enabling the organization and access of data through a file system. Volumes are essential for isolating storage resources, facilitating data integrity, and supporting operations like backup and recovery.[1] Volumes serve as a logical abstraction layer over physical hardware, decoupling the file system's structure from the specifics of the underlying storage devices like disks or solid-state drives. This abstraction allows the operating system to treat disparate or fragmented physical components as a unified whole, promoting efficient resource allocation and simplifying management tasks such as resizing or mirroring without direct hardware intervention. By hiding hardware boundaries, volumes enable greater flexibility in how data is stored, accessed, and maintained across diverse storage configurations.[9] The concept of volumes originated in early operating systems of the 1960s, with pioneering implementations in systems like Multics, which emphasized comprehensive storage management for time-sharing environments, and IBM's OS/360, which introduced volume serial numbers for unique identification of direct-access storage devices (DASD). These early designs laid the foundation for modern storage flexibility, evolving to support virtualized and distributed systems while maintaining core principles of abstraction and manageability.[10][11] At a high level, the lifecycle of a volume begins with creation, typically through initialization or formatting of the storage medium to establish its structure and assign a unique identifier. Once created, the volume is mounted—integrated into the operating system's namespace to become accessible for reading and writing data. When operations conclude, the volume is unmounted to prevent data corruption, allowing safe removal, reconfiguration, or reassignment of the underlying resources; partitions often serve as foundational building blocks in this process.[1]Relation to Storage Devices
In computing, a storage volume serves as a logical abstraction that maps to underlying physical hardware, enabling data organization across one or more storage devices. This mapping allows volumes to utilize portions of a single disk or extend across multiple disks through techniques such as concatenation, where unallocated space from different drives is combined sequentially to form a single contiguous volume, or striping, which distributes data in parallel stripes across disks to enhance read/write performance.[12][13] For instance, in RAID-0 configurations, a striped volume breaks data into blocks and writes them across multiple physical disks in a round-robin manner, improving throughput for large sequential operations without providing redundancy.[14] At the foundational level, volumes relate to the storage hierarchy through block devices, which provide random access to data in fixed-size units known as blocks or sectors—the smallest addressable elements on a disk, traditionally 512 bytes but increasingly 4 KiB in modern drives for efficiency.[15][16] Historically, storage geometry was described using cylinders (concentric groups of tracks), tracks (concentric circles on a platter), and sectors per track, though contemporary systems primarily employ Logical Block Addressing (LBA) to abstract these physical details and present a linear sequence of sectors to the volume layer.[17] In virtualized environments, volumes can be fully software-defined, decoupling them from direct physical hardware mappings. Hypervisors such as VMware vSphere introduce Virtual Volumes (vVols), which represent object-level storage entities managed by storage arrays over networked infrastructures like Fibre Channel or iSCSI, allowing policy-driven provisioning and granular control without traditional LUN (Logical Unit Number) boundaries.[18] This approach enables dynamic scaling in cloud or data center settings, where volumes aggregate resources from distributed storage pools.[18] Modern storage systems distinguish between static and dynamic volumes to support flexibility. Static volumes, akin to fixed partitions on basic disks, have predefined sizes that cannot be altered without reformatting, limiting adaptability.[12] In contrast, dynamic volumes in frameworks like Linux's Logical Volume Manager (LVM) permit online resizing—extending or shrinking without unmounting—by adjusting mappings to underlying physical extents, facilitating efficient resource allocation in growing environments.[19] Similarly, Windows dynamic disks support such resizable volumes, including spanned types that concatenate space across drives.[20]Distinctions from Related Terms
Volume vs. Partition
In computing storage, a partition refers to a fixed, contiguous division of physical storage space on a disk drive, typically created during the disk formatting process using tools like fdisk or Disk Management.[21][12] These divisions are defined by a partition table (such as MBR or GPT) that specifies the starting and ending sectors, allowing the disk to be segmented into independent areas for data isolation.[5] A volume, in contrast, is a logical construct that represents an accessible storage unit, often built upon one or more partitions and formatted with a file system (e.g., NTFS or ext4) to enable data storage and retrieval.[12][21] Unlike partitions, which are strictly tied to the physical layout of a single disk, volumes can aggregate multiple partitions, extend across disks, or be dynamically resized, providing abstraction from the underlying hardware.[5][12] The primary conceptual difference lies in their scope and flexibility: partitions serve as the foundational physical subdivisions that cannot inherently span multiple disks or be easily reconfigured without reformatting, whereas volumes act as higher-level logical entities that can combine or redistribute storage resources for more efficient management.[21][22] For instance, in systems using Logical Volume Manager (LVM) on Linux or dynamic disks on Windows, a volume can pool space from several partitions into a unified logical space, enabling features like spanning or mirroring that are impossible with standalone partitions.[5][12]| Aspect | Partition | Volume |
|---|---|---|
| Nature | Physical division on a single disk | Logical abstraction, often spanning partitions or disks |
| Creation | Via partitioning tools (e.g., fdisk, Diskpart) | Via volume managers (e.g., LVM, dynamic disks) |
| Flexibility | Fixed size and location; resizing requires repartitioning | Dynamically resizable without downtime |
| Scope | Limited to one disk | Can aggregate multiple disks |
Volume vs. Logical Drive
In the context of Windows operating systems, a logical drive typically refers to a partition located within an extended partition on a basic disk formatted using the Master Boot Record (MBR) partitioning scheme, enabling users to create additional storage units beyond the MBR's limit of four primary partitions.[23][24] This structure was common in older systems to subdivide disk space efficiently, where logical drives inherit drive letters and function similarly to primary partitions but require an enclosing extended partition.[12] The term "volume," however, serves as a more general designation for any discrete logical storage unit managed by the file system, including primary partitions, logical drives on basic disks (known as basic volumes), and configurable units on dynamic disks (known as dynamic volumes).[25][22] While logical drives are predominantly a Windows-specific construct tied to basic disk partitioning for handling subdivided spaces, volumes represent a platform-agnostic abstraction of storage that can aggregate or virtualize physical resources across operating systems.[2][26] This distinction has evolved alongside advancements in disk partitioning standards; the shift from MBR to the GUID Partition Table (GPT) in modern operating systems eliminates the need for extended partitions and logical drives by supporting up to 128 primary partitions directly, promoting the broader "volume" terminology to accommodate larger, more flexible storage configurations exceeding 2 terabytes.[12][27] In Windows environments, for instance, the C: drive can operate as either a logical drive or a basic volume depending on the partitioning setup, but the volume model facilitates dynamic operations like spanning multiple disks or mirroring without rigid repartitioning constraints.[22] Underlying these concepts are disk partitions, which form the foundational divisions on physical storage devices.[14]Operating System Nomenclature
Windows Systems
In Windows operating systems, volumes are typically referenced and accessed via drive letters, such as C: or D:, which act as mount points within the file system to provide a user-friendly abstraction for storage locations.[28] These letters are assigned to volumes during formatting or through system utilities, allowing applications and users to interact with the underlying storage without directly referencing physical devices.[29] By default, the system drive is assigned C:, while subsequent volumes receive sequential letters, though this can be customized to avoid conflicts with removable media.[30] Windows supports dynamic disks as an advanced storage management feature, enabling the creation of various volume types beyond basic partitioning. Simple volumes reside on a single dynamic disk and function similarly to basic partitions, while spanned volumes combine unallocated space across multiple disks to create a single logical volume larger than any individual disk.[12] Striped volumes distribute data across multiple disks for improved read/write performance without redundancy, and mirrored volumes duplicate data on two disks to enhance fault tolerance.[2] These configurations require converting basic disks to dynamic, which uses the Logical Disk Manager to handle volume metadata.[22] The Disk Management utility, integrated into Windows since Windows 2000, serves as the primary graphical tool for volume operations, including assigning drive letters, creating dynamic volumes, and converting disk types without detailed procedural instructions.[31] It displays volumes as horizontal bars representing their layout on disks and supports both basic and dynamic configurations for comprehensive storage oversight.[32] Historically, Windows 95 relied on the File Allocation Table (FAT) file system for volumes, which supported basic formatting but lacked advanced features like journaling.[33] The New Technology File System (NTFS), introduced with Windows NT 3.1 in 1993 and adopted in consumer versions starting with Windows 2000, brought improvements such as file compression, encryption, and quotas to volumes.[33] In modern Windows, including Server 2012 and later, the Resilient File System (ReFS) supplements NTFS for volumes requiring high scalability and data integrity, particularly in server environments.[34] Additionally, the Volume Shadow Copy Service, available since Windows XP and Server 2003, allows creation of shadow copies—point-in-time snapshots of volumes—for backup and recovery without interrupting system operations.[35] Unlike Unix-like systems that reference volumes via device paths like /dev/sda1, Windows emphasizes drive letters for intuitive access.[12]Unix-like Systems
In Unix-like operating systems, storage volumes are primarily represented through device files located in the/dev directory, which serve as interfaces to block devices such as hard disks and partitions. For example, SCSI or SATA disks are typically denoted as /dev/sdX (where X is a letter like a for the first disk), with partitions appended as /dev/sdXY (e.g., /dev/sda1 for the first partition on the first disk). NVMe drives are denoted as /dev/nvmeCnNsN for the namespace (e.g., /dev/nvme0n1), with partitions as /dev/nvmeCnNsNpP (e.g., /dev/nvme0n1p1).[36][37][38] For more complex setups like logical volumes managed by the Logical Volume Manager (LVM), devices appear under /dev/mapper/ paths, such as /dev/mapper/vg_name-lv_name.[19]
Volumes in Unix-like systems are accessed by mounting them to directories within the hierarchical file system, rather than assigning drive letters as in Windows. This process integrates the volume's file system into the overall directory tree, for instance, mounting a volume at /mnt/[data](/page/Data) to make its contents available via that path.[39] Manual mounting uses the mount command (e.g., mount /dev/sda1 /mnt/[data](/page/Data)), while automatic mounting at boot is configured via the /etc/[fstab](/page/Fstab) file, which specifies the device, mount point, file system type, and options for each entry.[40] Additionally, tools like autofs enable on-demand mounting for removable or network volumes, reducing resource overhead by only attaching them when accessed.[41]
The Logical Volume Manager (LVM) provides advanced volume management by abstracting physical storage into flexible logical units, allowing volumes to span multiple disks without rigid partitioning. LVM operates in layers: physical volumes (PVs) are initialized from whole disks or partitions (e.g., via pvcreate /dev/sda), which are then grouped into volume groups (VGs) using vgcreate to pool storage capacity. Logical volumes (LVs) are created from VG space with lvcreate, enabling features like resizing, snapshots, and striping for improved performance and redundancy.[5][42]
Variants of volume handling differ across Unix-like systems to suit their ecosystems. In Linux distributions, volumes often use file systems like ext4, formatted on partitions or LVs and mounted as described, providing robust support for large-scale storage with journaling for data integrity.[37] macOS, based on a Unix-like kernel, employs the Apple File System (APFS) since macOS High Sierra, where volumes reside within containers that share free space across multiple volumes (e.g., system, data, and recovery volumes in a single container), enabling efficient snapshots and encryption.[43][44] In BSD systems like FreeBSD, volume management leverages the GEOM framework for modular transformations, with ZFS serving as an integrated volume and file system that pools storage into datasets and volumes, supporting features like RAID-Z and deduplication natively.[45][46]